This invention relates to a circuit element utilizing magnetostatic wave produced by magnetic spin resonance of a thin film of a magnetic material, such as, YIG (yttrium iron garnet) formed on a substrate of a non-magnetic material, such as, GGG (gadolinium gallium garnet), and more particularly to the structure of such a circuit element in which an unnecessary spurious mode is suppressed so that the circuit element can operate over a wide frequency band.
A ferrimagnetic thin-film resonance element has been proposed as a circuit element suitable for use in, for example, a microwave oscillator circuit. In such a circuit element, a thin film of YIG (yttrium iron garnet) epitaxially grown from a liquid phase on a non-magnetic substrate of GGG (gadolinium gallium garnet) is shaped into a desired pattern, as disclosed in, for example, JP-A-2-13101.
This ferrimagnetic thin-film resonance element has such various features that the value of Q of its resonance characteristic in the microwave frequency band is high, its resonance frequency is variable according to the strength of a DC biasing magnetic field applied in a direction perpendicular to the ferrimagnetic thin film magnetically coupled to a microwave transmission channel (such as, a finger-shaped electrode formed by etching), etc.
As a resonator utilizing the ferrimagnetic thin-film resonance described above, a circuit element utilizing magnetostatic wave has been proposed so as to facilitate adjustment of the magnetic coupling between the ferrimagnetic thin film and the microwave transmission channel and to also improve the degree of magnetic coupling between the ferrimagnetic thin film and the microwave transmission channel. In this circuit element, the microwave transmission channel is formed on the ferrimagnetic thin film by the technique of photoetching, as disclosed in, for example, JP-A-62-245704 corresponding to U.S. Pat. No. 4,743,874.
FIG. 2A schematically shows the structure of the prior art circuit element utilizing magnetostatic wave, and FIG. 2B schematically shows the structure of a prior art magnetostatic wave resonator 6 used in the circuit element shown in FIG. 2A. The prior art magnetostatic wave resonator 6 shown in FIG. 2B includes a thin film 3 of YIG formed on a substrate 2 of GGG by the method of epitaxial growth from a liquid phase, one or a plurality of finger-shaped electrodes 5 each in the form of a film of Au or Al formed by the technique of photoetching on the thin film 3 of YIG, and pad electrodes 4a and 4b formed by the technique of photoetching on both sides respectively of the group of the finger-shaped electrodes 5.
As shown in FIG. 2A, a conductive surface of a micro stripline 15 is partly removed, and a conductor strip 11 and an impedance matching stub 7 are formed on both sides respectively of the removed part g of the conductive surface of the micro stripline 15. Thus, the micro stripline 15 is disconnected at the gap g in a DC sense. After the magnetostatic wave resonator 6 shown in FIG. 2B is fixed to the area of the gap g of the micro stripline 15, the conductor strip 11 is electrically connected to the pad electrode 4a by a connector strip 12a, and the impedance matching stub 7 is electrically connected to the pad electrode 4b by a connector strip 12b to complete the circuit element 16 utilizing the magnetostatic wave.
When the transmission spectrum of the circuit element 16 utilizing the magnetostatic wave was measured, it was frequently observed that a spurious mode tended to appear in the vicinity of the lowest order resonance mode of the circuit element 16 as shown in FIG. 4. If all of these modes were attributable to the magnetostatic wave, the waveform shown in FIG. 4 should shift while maintaining the same form regardless of any variation in the strength of an externally applied magnetic field. However, in the case of the spurious mode, its resonance frequency sensitivity with respect to the variation in the strength of the externally applied magnetic field slightly differed in many cases from that of the lowest order resonance mode of the circuit element, and the position of the spurious mode relative to the lowest order resonance mode of the circuit element tended to change depending on the resonance frequency. Therefore, in the case of the spurious mode, it is likely that the resonance is not dependent on transmission of pure magnetostatic wave.
More concretely, when the resonance spectrum of the prior art circuit element utilizing the magnetostatic wave was observed while changing the resonance frequency at the lowest order resonance mode by gradually changing the strength of the externally applied magnetic field, the spurious mode observed on one side of the peak of the resonance at the lowest order resonance mode gradually approached the peak and then passed the peak to shift toward the other side of the peak. (Refer to IEEE Transaction on Magnetics, Vol.Mag-20, No.5, September 1984.)
When these spurious modes overlapped the lowest order resonance mode, the peak at the lowest order resonance mode became dull, and the problems including a great reduction of the value of Q indicating the sharpness of resonance have resulted.